The present invention relates to integrated circuits used in radio frequency applications. Specifically, a fully integrated voltage controlled oscillator (VCO) is provided having reduced second harmonic content and improved phase noise performance.
Radiotelephone communications such as cellular telephone applications rely on analog circuits for generating various channel frequencies. The current design practices are to implement voltage controlled oscillators as part of a frequency synthesizer application to generate the various closely spaced signal channels for transmission and reception. These circuits typically rely upon a tuned circuit which has a digital frequency control to select a frequency band of operation, as well as a voltage tuned varactor to select a precise frequency within the selected band. Once the frequency, band is selected digitally, the oscillator operates as a voltage controlled oscillator by varying the voltage applied to the varactor.
A common circuit for providing a voltage controlled oscillator is described in xe2x80x9cA Low Phase Noise Monolithic VCO in SiGe BiCMOS,xe2x80x9d 2000 IEEE Radio Frequency Integrated Circuit Symposium. These devices are implemented using cross-coupled bipolar transistors with a tuned circuit in the collectors of the cross-coupled transistors. The high phase noise level in the voltage controlled oscillator (VCO) embedded in the frequency synthesizer, is one of the major limitations to the full integration of transceivers for wireless application. One of the contributors to the phase noise are second harmonic signals which are generated by the current source which supplies currents to the emitter connections of the cross-coupled bipolar transistors. The second harmonic signals in turn generated mixing products in the tuned circuit of the VCO which is seen as phase noise in the output signal of the VCO. A net improvement in phase noise performance can therefore be obtained if second harmonic generation in the circuit can be suppressed.
It is an object of the present invention to provide a fully integrated voltage controlled oscillator having improved phase noise performance.
This and other objects of the invention are provided for by a cross-coupled differential bipolar transistor voltage controlled oscillator. The voltage controlled oscillator includes digital tuning means for selecting one of the plurality of frequency bands which are to be tuned by a pair of varactor diodes using an analog control voltage. A current source supplies an operating current to the transistors. Circuitry is provided for suppressing second harmonic signals generated by the current source which effectively eliminates one of the significant sources of phase noise in the circuit.
In one embodiment of the invention, a filter trap having a resonant frequency centered at the second harmonic signal frequency is placed across the current source to eliminate, or substantially reduce, the second harmonic signal amplitude. In accordance with a preferred embodiment of the invention, the tuned filter trap is a series resonant LC circuit having a center frequency set to be substantially within the frequency band of second harmonics generated by the voltage controlled oscillator. In a multiband application, traps having different. resonant frequencies, corresponding to the different second harmonic frequencies for different bands, may be digitally selected when a band of operation is selected.